High pressure, high temperature gasification systems have been used to partially oxidize hydrocarbonaceous fuels to recover useful by-products or energy. The fuels can be admixed with water to form an aqueous feedstock that is fed to the reaction zone of a partial oxidation gasifier along with a oxygen containing gas and a temperature moderator.
Mixing the feed with water may not be necessary, given the composition and physical nature of the feedstock. Generally, solid carbonaceous fuels will need to be liquefied with oil or water prior to feeding to the gasifier. Liquid and gaseous hydrocarbonaceous fuels may be suitable for direct feed to the gasifier, but can be pre-treated for removal of any impurities that might be present in the feed.
The term liquid hydrocarbonaceous fuel as used herein to describe various suitable feedstocks is intended to include pumpable liquid hydrocarbon materials and pumpable liquid slurries of solid carbonaceous materials, and mixtures thereof. In fact, any combustible carbon-containing liquid organic material, or slurries thereof may be included within the definition of the term xe2x80x9cliquid hydrocarbonaceousxe2x80x9d. For example, there are pumpable slurries of solid carbonaceous fuels, liquid hydrocarbon fuel feedstocks, oxygenated hydrocarbonaceous organic materials, and mixtures thereof. Gaseous hydrocarbonaceous fuels may also be burned in the partial oxidation gasifier alone or along with liquid hydrocarbonaceous fuel.
The partial oxidation reaction is preferably carried out in a free-flow, unpacked non-catalytic gas generator. Under high temperature and high pressure conditions, about 98% to 99.9% of the hydrocarbonaceous feedstock can be converted to a synthesis gas containing carbon monoxide and hydrogen, also referred to as synthesis gas or syngas. Carbon dioxide and water are also formed in small amounts.
Water is further used as quench water to quench the syngas. This quench water is also used to scrub particulate matter from the syngas and to cool and/or convey particulate waste solids, such as ash and/or slag out of the gasifier. In order to conserve water, gasification units reuse most of the quench water. A portion of the water is normally continuously removed as an aqueous effluent, grey water, purge wastewater or blowdown stream to prevent excessive buildup of solid materials and undesired dissolved solids.
The composition of the grey water discharged from the gasification system is fairly complex. This water can contain chlorides, ammonium salts, and other potentially environmentally harmful dissolved materials such as sulfide and cyanide. Thus, the effluent wastewater from the gasification system cannot be discharged to the environment without treatment and solids removal.
The grey water blowdown stream is discharged from the gasification system, and is treated with chemicals to precipitate impurities in the grey water. For example, Ferrous Sulfate (FeSO4) can be added to produce an iron hydroxide floc (Fe(OH)2) to remove any sulfide, cyanide and particulate matter. This process is usually done in a combination rapid mix reactor and solids settler. The chemicals are added to the rapid mix reactor where they are mixed with the grey water. The effluent from the rapid mix reactor is sent to the solids settler, where any precipitated solids and particulate matter are allowed to settle out of the grey water. After having the solids removed, the grey water can be subjected to ammonia stripping, biological treatment, or evaporation to produce a dry salt for commercial marketing and a distillate water. The water can then be recycled to the gasification quench process thereby eliminating any wastewater discharge from the plant.
Referring first to FIG. 1, a common prior art rapid mix reactor/solids settler integrated system is shown. Grey water from a gasification unit (not shown) is fed through line 10 to rapid mix reactor 14. Chemicals are also added to the rapid mix reactor 14 through any of lines 12. In rapid mix reactor 14, the grey water and the chemicals form a liquid level 22, that is stirred by mixer 16 which is driven by motor 18. Four baffle plates inside the rapid mix reactor provide thorough mixing. Rapid mix reactor 14 has a level indicator 20 which sends a signal to level controller 21, which is used to keep the level 22 of the grey water and chemicals as constant as possible.
The rapid mix reactor 14 effluent leaves out the bottom of the rapid mix reactor 14 through line 24 into the coagulation chamber 26 of solids settler 30. The coagulation chamber 26 is defined by a circular wall 28 in solids settler 30. In coagulation chamber 26 the solids and precipitates in the grey water are allowed to fall out of solution. The solids fall to the conical shaped bottom 32 of the solids settler 30, where they are removed via line 34 and are sent to a filter press (not shown).
Generally solid-free grey water, otherwise known as clarified water, leaves out the top of the solids settler 30 through line 36. Control valve 38 is positioned in line 36, and is controlled by level controller 21. After passing through control valve 38, the clarified water is sent through line 40 to a downstream processing unit for further treatment, usually an alkalization reactor (not shown).
In this prior art scheme of integrating the rapid mix reactor and the solids settler, the rapid mix reactor effluent flows out of the bottom of the rapid mix reactor 14 through line 24 and enters the center of the solids settler 30. Flow controller 38 is located in the clarified water outlet line 36 from the solids settler 30. The signal line of this flow controller 38 is connected to the level controller 21 of the rapid mix reactor 14, the objective of such an arrangement being to maintain the liquid level of grey water and chemicals in the rapid mix reactor 14. The failure of instrumentation in the rapid mix reactor 14 that could cause a high or low level in the rapid mix reactor 14 would cause the control valve 38 to have extreme swings from open to closed, sometimes causing the control valve 38 to swing wide open or go completely closed. This causes a harmful effect in the overall grey water treatment process. It can cause a low level in the downstream processing units, which can initiate the interlock system of those processing units which in turn shuts down those units, as well as the whole grey water treatment process.
Usually the combination of the rapid mix reactor and the solids settler is the first in many grey water process treatments. Common prior art systems such as the one described above control the integration of the rapid mix reactor and the solids settler in such a manner that can cause problems and process upsets in downstream process units. Thus, it would be desirable to develop an integrated rapid mix reactor/solids settler system that efficiently removes solids and precipitates from the grey water while minimizing the possibilities of causing upsets in downstream grey water process equipment.
The present invention involves an improved integrated reactor and solids settler system for solids removal from a water stream. The integration of the rapid mix reactor and the solids settler of the present invention uses an overflow line from the rapid mix reactor to eliminate both the rapid mix reactor level indicator and controller and the flow controller in the solids settler clarified water outlet. The proposed design is simple, cost effective, and eliminates the possibility of unnecessary downstream unit shutdowns.